You can find more problems and solutions like these in the book "Practice Problems in Physics" by Abhay Kumar.
Given $u = 20$ m/s, $g = 9.8$ m/s$^2$
$= 6t - 2$
$\Rightarrow h = \frac{400}{2 \times 9.8} = 20.41$ m practice problems in physics abhay kumar pdf
Given $v = 3t^2 - 2t + 1$
At $t = 2$ s, $a = 6(2) - 2 = 12 - 2 = 10$ m/s$^2$ You can find more problems and solutions like
$0 = (20)^2 - 2(9.8)h$